Article pubs.acs.org/est
Optical Properties of the Products of α-Dicarbonyl and Amine Reactions in Simulated Cloud Droplets Kyle J. Zarzana,†,‡ David O. De Haan,‡,§ Miriam A. Freedman,‡,∥ Christa A. Hasenkopf,‡,⊥ and Margaret A. Tolbert†,‡,* †
Department of Chemistry and Biochemistry and ‡Cooperative Institute for Research in Environmental Sciences (CIRES), Department of Atmospheric and Oceanic Sciences, University of Colorado at Boulder, Boulder, Colorado 80309-0216, United States § Department of Chemistry and Biochemistry, University of San Diego, 5998 Alcala Park, San Diego, California, 92110 ⊥
ABSTRACT: Secondary organic aerosol makes up a significant fraction of the total aerosol mass, and a growing body of evidence indicates that reactions in the atmospheric aqueous phase are important contributors to aerosol formation and can help explain observations that cannot be accounted for using traditional gas-phase chemistry. In particular, aqueous phase reactions between small organic molecules have been proposed as a source of light absorbing compounds that have been observed in numerous locations. Past work has established that reactions between α-dicarbonyls and amines in evaporating water droplets produces particle-phase products that are brown in color. In the present study, the complex refractive indices of model secondary organic aerosol formed by aqueous phase reactions between the α-dicarbonyls glyoxal and methylglyoxal and the primary amines glycine and methylamine have been determined. The reaction products exhibit significant absorption in the visible, and refractive indices are similar to those for light absorbing species isolated from urban aerosol. However, the optical properties are different from the values used in models for secondary organic aerosol, which typically assume little to no absorption of visible light. As a result, the climatic cooling effect of such aerosols in models may be overestimated.
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INTRODUCTION Atmospheric aerosol particles play a significant but poorly understood role in affecting the Earth’s radiative balance. Aerosol particles can directly interact with light by scattering or absorbing incoming radiation, and the amount of scattering or absorption determines whether aerosol particles have a net warming or cooling effect on climate.1,2 The amount of light that a substance scatters versus absorbs can be quantified by the complex refractive index
to form aerosol. The compounds that are formed by this route are typically assumed in models to be non or only slightly absorbing in the visible; that is they have a k value close or equal to zero.4−6 However, there is growing evidence that a substantial amount of OA does absorb in the visible, particularly at shorter wavelengths. These compounds have been termed “brown carbon”7 and have been observed in both urban8−10 and rural10−12 environments. Recent reviews have pointed to the role that aqueous chemistry can play in increasing aerosol mass13,14 and the products of many of these reactions, including reactions on acidic particles,15 aldol condensation reactions,16 and reactions with various nitrogencontaining species,17 produce species that absorb visible light, in addition to numerous nonabsorbing compounds. Much of the work on aqueous aerosol chemistry has focused on the role of glyoxal and methylglyoxal. Glyoxal and methylglyoxal are the smallest α-dicarbonyls and due to their high volatility cannot condense into the organic phase of particles through traditional gas-particle partitioning. However, both readily partition to the aqueous phase and have been shown to react with a number of species to form lightabsorbing compounds.18−20 De Haan et al.21−23 have shown that glyoxal and methylglyoxal can undergo aqueous phase
(1)
m = n + ik
where n, the real part, describes the scattering, whereas k, the imaginary part, describes the absorption. The complex refractive index of aerosol particles is a useful quantity to measure because unlike other means of quantifying scattering and absorption, such as the single scatter albedo, the refractive index depends primarily on composition and does not depend on particle size or concentration. Using the refractive index and the particle shape, secondary quantities such as the absorption and scattering cross sections and the single scatter albedo can be calculated. Globally, organic compounds make up 50% of aerosol mass, and in polluted areas this amount can be as high as 90%.3 Given the ubiquity and dominance of organic aerosol (OA), understanding the optical properties of OA is crucial to predicting its effects on radiative forcing. The traditional formation mechanism of OA is through the gas phase oxidation of volatile organic compounds (VOCs), which then condense © 2012 American Chemical Society
Received: Revised: Accepted: Published: 4845
November 10, 2011 March 21, 2012 April 9, 2012 April 19, 2012 dx.doi.org/10.1021/es2040152 | Environ. Sci. Technol. 2012, 46, 4845−4851
Environmental Science & Technology
Article
reactions with primary amines, producing a variety of compounds including oligomers and imidazoles. These compounds contain carbon−nitrogen bonds, a feature commonly observed in ambient aerosol.24,25 Additionally, the product mixtures are brown in color, and therefore contribute to light absorption in the visible. The present study builds on previous work that examined the formation kinetics and aerosol products of aqueous phase reactions.21−23 The goal of the present study is to quantify the optical properties of model secondary organic aerosol formed by these aqueous phase reactions. The refractive indices of compounds from four model systems were determined and then compared to field measurements of brown carbon aerosol. Using the calculated refractive indices, the single scattering albedo and the relative forcing in the visible of the particles were also calculated. Additionally, estimates of the cooling effect of these particles and particles used in climate models are presented.
Figure 1. Setup for cavity ring-down experiments. Four driers were used in the experiments, but only the two diffusion driers are shown for simplicity.
(R > 99.998%, Advanced Thin Films). The light that enters the cavity reflects off the mirrors numerous times, and the time decay of the signal that is transmitted out the other end is measured with a photomultiplier tube (PMT, Hamamatsu HC120). The decay time is measured under conditions when there is no aerosol present in the cavity (τ0) and when the aerosol is present (τ). The extinction, αext, (with dimensions of inverse length) is then calculated from
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EXPERIMENTAL SECTION Four model systems were studied: glyoxal-glycine; glyoxalmethylamine; methylglyoxal-glycine; and methylglyoxal-methylamine. Solid glyoxal trimer dihydrate, solid glycine, 40% w/w aqueous methylamine (all Sigma Aldrich), and 40% w/w aqueous methylglyoxal (MP Biomedicals) were used without further purification. Samples were generated in a manner similar to the bulk phase samples used by De Haan et al.21−23 One M stock solutions of each were created, and then 150 μL of an α-dicarbonyl (either glyoxal or methylglyoxal) was combined with 150 μL of an amine (either glycine or methylamine) in small vials and allowed to dry for several days at room temperature. These concentrations were chosen in order to be consistent with the concentrations used by De Haan et al.,21−23 and we note that while the concentrations of dicarbonyls and amines vary depending on location, these compounds can exist at roughly equal concentrations, particularly in urban areas.26,27 Additionally, while this reaction time is longer than the lifetime of an atmospheric cloud droplet, the rate of these reactions is significantly increased by the drying process, and analysis of mass spectrometer data of products from both bulk reactions and simulated cloud droplet reactions indicate that the products are chemically similar.21,22 The resulting brown product was then dissolved in HPLCgrade water, and atomized (TSI 3076) using prepurified nitrogen. The aerosol was passed through two custom-made 4 L Erlenmeyer flask driers and two diffusion driers (TSI model 3062) to remove the water and reduce the relative humidity (RH) to
